A wide variety of OLEDs are known. Some OLEDs, referred to as “bottom emitting” OLEDS, emit light through a transparent substrate on which the OLED is fabricated. Others, referred to as “top emitting” OLEDs, emit light in the opposite direction, i.e., away from the substrate on which the OLED is fabricated. Some OLEDs are patterned to form an array of individually addressable OLED emitters, referred to individually as pixels (picture elements) or subpixels (one of several neighboring emitters of different colors that are grouped together as a pixel but are individually addressable). Such pixelated OLEDs are becoming increasingly popular for use in digital display devices such as for mobile phones and similar end uses. In contrast to pixelated OLEDs, other OLEDs are designed to have only one emitting area, which may be small and narrow or large and extended depending on the intended application.
One issue of concern to some OLED manufacturers and designers is the less-than-ideal efficiencies exhibited by OLEDs due to peculiarities of their design. The external efficiency of an OLED, or of any other self-emissive light source, can be calculated as the power of all optical radiation emitted by the device divided by the total electrical power consumed by the device. OLED external efficiency is a significant design parameter for many different OLED applications, ranging for example from pixelated OLEDs used in high-resolution displays to non-pixelated OLEDs used in lighting systems, since the external efficiency affects such device characteristics as power consumption, luminance, and lifetime. A number of groups have demonstrated that OLED external efficiency is significantly limited by optical losses within the active emitting layers of the OLED stack itself (due to waveguiding modes within high refractive index organic layers and indium tin oxide), within intermediate-refractive index substrates, and finally due to exciton quenching at the cathode (anode) metal's surface plasmon polaritons. In an OLED device that exhibits the maximum possible internal efficiency (i.e., 100% internal efficiency), about 75-80% of the generated optical radiation is dissipated internally due to the above-mentioned losses, resulting in a corresponding reduction in external efficiency. If additional optical components such as color filters or circular polarizers are included as elements of the OLED device, such components can produce additional optical losses and even lower external efficiencies.
Some optical films have already been proposed to enhance light extraction from OLED devices. Reference in this regard is made to U.S. Patent Application Publications US 2009/0015142 (Potts et al.), US 2009/0015757 (Potts et al.) (now issued as U.S. Pat. No. 8,179,034), and US-2010/0110551 (Lamansky et al.), which are incorporated herein by reference in their entireties.